Traditionally Light Emitting Diodes (LEDs) have primarily been used as indicator lamps in electronic equipment. However recently the power and efficacy (e.g., lumens per watt of electrical power) has been increasing and LEDs have been identified as a possible replacement for inefficient incandescent lamps in certain applications. The light emitting region of an LED is small (e.g., in the range of 2 mm to 0.5 mm across in many cases) which in theory opens up the possibility for highly controlled distribution of light. However many of LED optics developed so far do not produce controlled distributions, rather they typically produce Gaussian like distributions which is the hallmark of somewhat uncontrolled (random) light distribution, and is not ideal for most, if not all applications.
FIG. 1 is a graph including plots of light intensity versus polar angle for an ideal Lambertian (cosine) source 102, and a quasi-Lambertian white LED with a hemispherical primary lens 104. The intensity plot for the white LED is based on measurements taken at distance about 300 mm beyond the hemispherical primary lens surface. Note that the actual white LED distribution is close to the ideal Lambertian distribution. The Lambertian distribution is not particularly suited to any illumination tasks. Lens described herein below are able to redistribute light and produce more useful distributions. Note, as will be explained below, lenses taught herein can be adapted to LEDs, e.g., colored LEDs, that produce light distributions that depart more significantly from the Lambertian distribution shown in FIG. 1. This is because the lenses described herein below have shapes defined in terms of the light distribution of the LED (or other source) for which they are designed and the desired altered light distribution. It is noted that there is some uncertainty as to the exact angular distribution of light produced by a bare chip within its encapsulating primary lens, e.g., whether it might be closer to curve 102 or 104 however it is matter of minor consequence as the two distribution are sufficiently close that either might be used and the resulting distribution can be corrected with a few prototype iterations (as described below).
FIG. 2 shows a reflector 202 arranged to collect a portion of light emitted by an LED 204. A problem with using a reflector with an LED that emits over the entire hemisphere of solid angle is that the reflector needs to have an aperture and thus can not intercept and redirect all of the light. As shown in FIG. 1 light emitted within polar angle range from zero to φ passes through the aperture of the reflector 202 without redirection or control. Additionally for the reflector 202 to exert detailed control over the emitted light distribution it must be specular as opposed to diffuse, and polishing a reflector sufficiently to make it specular is often expensive.
In an attempt to address the problem posed by the hemispherical range of light output from LED, a type of “primary” optic 302 shown in FIG. 3 has been developed. (This is termed a “primary” optic because it is assumed that it may be used in conjunction with a “secondary” optic such as the reflector 202.) The term “primary optic” may also be taken to mean an optic which has an optical medium of index >1 extending from the LED die so that there only an outer optical surfaces. The primary optic 302 is designed to intercept light emitted by an LED chip which is positioned in a space 304 at the bottom of the primary optic 302 and to redirect the light radially outward, perpendicular to an optical axis 306. The primary optic includes a refracting part 308 and a TIR (Total Internal Reflection) part 310 both of which contribute to redirecting the light. One drawback of the primary optic 302 is that because it includes multiple optical surfaces that contribute to light in the same direction it will increase the effective size of the source (also the étendue), which reduces the controllability of light from the LED. The increased effective size of the source can in some cases be compensated for, by using larger secondary optics but this may be undesirable based on cost and space constraints. By way of loose analogy to imaging optics, the primary optic creates multiple “images” of the LED, e.g., one from the refracting part 308 and one from the TIR part 310.
Although, the primary optic 302 is intended to redirect light perpendicular to the optical axis, in practice light is redirected to a range of angles. This is because the primary optic is small and positioned in close proximity to the LED, and consequently the LED subtends a not-insignificant solid angle from each point of the primary optic, and light received within this finite solid angle is refracted or reflected into a commensurate solid angle. The result is shown in FIG. 4 which is a plot of light intensity vs. polar angle for an LED equipped with the primary optic 302. Although this distribution of light shown in FIG. 4 is not especially suited to any particular application, it is intended to direct light into an angular range that can be intercepted by a secondary optic e.g., reflector 202. The goal is not fully achieved in that the angular distribution of light produced by the primary optic 302 covers a range that extends from zero polar angle and therefore all of the light can not be intercepted by the reflector 202.
Another presently manufactured commercial optic 502 for LEDs is shown in FIG. 5. In use, an LED (not shown) will be located in a bottom recess 504. This optic 502 is one form of “secondary” optic. A LED with or without the primary optic 302 attached can be used. If used the primary optic will fit inside the bottom recess 504. The secondary optic 502 is made from optical grade acrylic (PMMA) and is completely transparent with no reflective coatings. The optic 502 includes a TIR (Total Internal Reflection) parabolic surface 506 which collects a first portion of light emitted by the LED, and a convex lens surface 508 which collects a remaining portion of the light. Both surfaces 506, 508 are intended to collimate light. As might be expected in actuality the light is distributed in a Gaussian like angular distribution over a certain angular range which is variously reported as 5 degrees and 10 degrees. The former value may be a FWHM value, and the actual value will vary depending on the exact LED that is used. This design is only useful for a fairly narrow range of specialized applications that require a far-field highly collimated LED spotlight. FIG. 6 shows an angular distribution of light produced by this type of optic. As shown the angular distribution is Gaussian-like not uniform.
In order to get a broader angular distribution of light some form of surface relief pattern can be added to a top surface 510 of the optic 502 which is planar as shown in FIG. 5. Alternatively, the surface relief pattern can be formed on a “tertiary” optic that is attached to the top surface 510. One type of surface relief pattern-concentric rings of convolutions is shown in a plan view in FIG. 7 and in a broken-out sectional elevation view in FIG. 8. Another type of surface relief pattern-an array of lenslets is shown in a plan view in FIG. 9 and in a broken-out sectional elevation view in FIG. 10. FIGS. 11 and 12 show light intensity distributions produced by commercial optics that have the same general design as shown in FIG. 5 but which have top surfaces with a surface relief pattern to broaden the angular distribution. The distribution shown in FIG. 11 is designated as having a 15 degree half-angle pattern and that shown in FIG. 12 a 25 degree half angle pattern.
Beyond the basic hemispherical primary lens other attempts have been made to obtain more useful distributions of light. FIG. 13 shows the profile of a primary lens designed by adding a spline perturbation to the basic hemispherical shape and optimizing the parameters of the spline perturbation using an optimization routine. One drawback of this lens is that it includes fine scale features that may be difficult to replicate in silicone which is often used to make LED primary lenses. Another drawback is that intensity distribution is sensitive to minute variations in the position of the LED die under the lens. FIG. 14 shows the irradiance pattern produced by the lens shown in FIG. 13 in two different planes. Another drawback of this lens is that the distribution is somewhat jagged. The jaggedness, which may arise due to the fine scale spline perturbation is not ideal.
The lens shown in FIG. 13 is representative of one approach to illumination optics which has been used with varying success during the last two decades-namely paramaterizing an optical surface in someway and using an optimization routine to vary the parameters while checking an objective function that depends on the pattern of light produced by the lens. Such optimization is generally considered a method of last resort, when a problem appears to be intractable.
FIG. 15 is a sketch of the profile of a “batwing” primary lens 1500 available on Luxeon™ LEDS made by Lumileds of San Jose, Calif. Batwing distributions, which predate the interest in using LEDs for lighting and were achieved by some fluorescent light fixtures for example, are characterized by increasing radiant intensity as a function of polar angle. The batwing primary lens shown in FIG. 15 appears to consist of a conical side wall 1502 joined to a relatively low curvature top surface 1504 by a radiused edge 1506. Arguably, the ideal batwing distribution is the theoretically known cos−3(φ) distribution, where φ is the polar angle. This distribution is ideal in the sense that it will illuminate a plane surface uniformly. FIG. 16 shows the actual distribution of light produced by the lens 300 (represented by measured data points) along with a plot of cos−3(φ). The actual distribution produced by the lens 1500 departs markedly from the cos−3(φ) distribution. Roughly speaking, the region around the radiused edge 306 creates a sort of positive lens which concentrates light in the direction of about 40 degrees so that the actual distribution exceeds the cos−3(φ) from about 20 to about 40 degrees and the sidewall 1504 allows a significant amount of light to form a tailing off of intensity up to about 70 degrees.
FIG. 17 shows another type of optic 1700 that is useful for illumination. This optic includes a saw tooth TIR section 1702 and a central lens portion 1704. The optic 1700 can collect a full hemisphere of emission from a source and forms an illumination pattern with a half-angle divergence (polar angle) about 30 degrees. This lens is disclosed in U.S. Pat. No. 5,577,492. For this type of optic there will be some loss of light from the intended distribution at the corners of the saw tooth pattern, which in practice may not be perfectly sharp due to manufacturing limitations. Additionally, due to its complex shape the cost of machining and polishing molds for injection molding is expected to be high. Additionally the '492 patent does address controlling the distribution of light within angular limits of the beams formed. The optic 1700 is already broad relative to its height. If an attempt were made to broaden the polar angle range of the illumination pattern, the TIR surfaces 1704 would have to be angled at larger angles, making the optic even broader-perhaps impractically broad.